when I ping to an non-exist network, the next hop router reply the icmp reply (unreachable messge) to me. however, I could only find 3 U messages only. Where are the missing 2 U messages?
I also used debug ip packet detail and receive 3 icmp type 3 messages. not 5 messages.
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 18.104.22.168, timeout is 2 seconds:
Success rate is 0 percent (0/5)
When you ping to a non existant address then the router that receives the ping request and realizes that the destination address is not reachable will generate an ICMP unreachable error message and send it to the originator of the ping. However Cisco routers rate limit their ping responses (as a mechanism to help protect against Denial of Service attacks against the router). The router is receiving 5 requests which can not be forwarded and sends the ICMP error to 3. The other 2 are rate limited.